446 research outputs found
Unresolved versus resolved: testing the validity of young simple stellar population models with VLT/MUSE observations of NGC 3603
CONTEXT. Stellar populations are the building blocks of galaxies including
the Milky Way. The majority, if not all extragalactic studies are entangled
with the use of stellar population models given the unresolved nature of their
observation. Extragalactic systems contain multiple stellar populations with
complex star formation histories. However, their study is mainly based upon the
principles of simple stellar populations (SSP). Hence, it is critical to
examine the validity of SSP models. AIMS. This work aims to empirically test
the validity of SSP models. This is done by comparing SSP models against
observations of spatially resolved young stellar population in the
determination of its physical properties, i.e. age and metallicity. METHODS.
Integral field spectroscopy of a young stellar cluster in the Milky Way, NGC
3603, is used to study the properties of the cluster both as a resolved and
unresolved stellar population. The unresolved stellar population is analysed
using the H equivalent width as an age indicator, and the ratio of
strong emission lines to infer metallicity. In addition, spectral energy
distribution (SED) fitting using STARLIGHT, is used to infer these properties
from the integrated spectrum. Independently, the resolved stellar population is
analysed using the color-magnitude diagram (CMD) for age and metallicity
determination. As the SSP model represents the unresolved stellar population,
the derived age and metallicity are put to test whether they agree with those
derived from resolved stars. RESULTS. The age and metallicity estimate of NGC
3603 derived from integrated spectroscopy are confirmed to be within the range
of those derived from the CMD of the resolved stellar population, including
other estimates found in the literature. The result from this pilot study
supports the reliability of SSP models for studying unresolved young stellar
populations.Comment: 9 pages, 5 figures, accepted to A&
GRB hosts through cosmic time. VLT/X-Shooter emission-line spectroscopy of 96 γ-ray-burst-selected galaxies at 0.1 <z < 3.6
We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 2 by ~0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts’ properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments
VLT/X-Shooter Near-Infrared Spectroscopy and HST Imaging of Gravitationally-Lensed z~2 Compact Quiescent Galaxies
Quiescent massive galaxies at z~2 are thought to be the progenitors of
present-day massive ellipticals. Observations revealed them to be
extraordinarily compact. The determination of stellar ages, star formation
rates and dust properties via spectroscopic measurements has up to now only
been feasible for the most luminous and massive specimens (~3x M*). Here we
present a spectroscopic study of two near-infrared selected galaxies which are
close to the characteristic stellar mass M* (~0.9x M* and ~1.3x M*) and whose
observed brightness has been boosted by the gravitational lensing effect. We
measure the redshifts of the two galaxies to be z=1.71\pm0.02 and
z=2.15\pm0.01. By fitting stellar population synthesis models to their
spectro-photometric SEDs we determine their ages to be 2.4^{+0.8}_{-0.6} Gyr
and 1.7\pm0.3 Gyr, respectively, which implies that the two galaxies have
higher mass-to-light ratios than most quiescent z~2 galaxies in other studies.
We find no direct evidence for active star-formation or AGN activity in either
of the two galaxies, based on the non-detection of emission lines. Based on the
derived redshifts and stellar ages we estimate the formation redshifts to be
z=4.3^{+3.4}_{-1.2} and z=4.3^{+1.0}_{-0.6}, respectively. We use the increased
spatial resolution due to the gravitational lensing to derive constraints on
the morphology. Fitting Sersic profiles to the de-lensed images of the two
galaxies confirms their compactness, with one of them being spheroid-like, and
the other providing the first confirmation of a passive lenticular galaxy at a
spectroscopically derived redshift z~2.Comment: accepted for publication in Ap
Detection of emission lines from z ~ 3 DLAs towards the QSO J2358+0149
Using VLT/X-shooter we searched for emission line galaxies associated to four
damped Lyman- systems (DLAs) and one sub-DLA at 2.73<=z<=3.25 towards
QSO J2358+0149. We detect [O III] emission from a "low-cool" DLA at z_abs =
2.9791 (having log N(HI)=21.69+\-0.10, [Zn/H] = -1.83+\-0.18) at an impact
parameter of, ~12 kpc. The associated galaxy is compact with a dynamical
mass of (1-6)x10^9 M_solar, very high excitation ([O III]/[O II] and [O
III]/[H] both greater than 10), 12+[O/H]<=8.5 and moderate star
formation rate (SFR <=2 M_solar yr^{-1}). Such properties are typically seen in
the low-z extreme blue compact dwarf galaxies. The kinematics of the gas is
inconsistent with that of an extended disk and the gas is part of either a
large scale wind or cold accretion. We detect Ly emission from the
z_abs = 3.2477 DLA (having log N(HI)=21.12+\-0.10 and [Zn/H]=-0.97+\-0.13).The
Ly emission is redshifted with respect to the metal absorption lines by
320 km s^{-1}, consistent with the location of the red hump expected in
radiative transport models. We derive SFR ~0.2-1.7 M_solar yr^{-1} and
Ly escape fraction of >=10 per cent. No other emission line is detected
from this system. Because the DLA has a small velocity separation from the
quasar (~500 km s^{-1}) and the DLA emission is located within a small
projected distance ( kpc), we also explore the possibility that the
Ly emission is being induced by the QSO itself. QSO induced Ly
fluorescence is possible if the DLA is within a physical separation of 340 kpc
to the QSO. Detection of stellar continuum light and/or the oxygen emission
lines would disfavor this possibility. We do not detect any emission line from
the remaining three systems.Comment: 13 pages, 7 figures, 4 tables (3 pages, 5 figures, 5 tables in
Appendix). Accepted for publication in MNRA
Dust reddening and extinction curves towards gamma-ray bursts at z > 4
Dust is known to be produced in the envelopes of AGB stars, the expanded
shells of supernova (SN) remnants, and in situ grain growth in the ISM,
although the corresponding efficiency of each of these dust formation
mechanisms at different redshifts remains a topic of debate. During the first
Gyr after the Big Bang, it is widely believed that there was not enough time to
form AGB stars in high numbers, so that the dust at this epoch is expected to
be purely from SNe, or subsequent grain growth in the ISM. The time period
corresponding to z ~5-6 is thus expected to display the transition from SN-only
dust to a mixture of both formation channels as we know it today. Here we aim
to use afterglow observations of GRBs at redshifts larger than in order
to derive host galaxy dust column densities along their line-of-sight and to
test if a SN-type dust extinction curve is required for some of the bursts. GRB
afterglow observations were performed with the 7-channel GROND Detector at the
2.2m MPI telescope in La Silla, Chile and combined with data gathered with XRT.
We increase the number of measured values for GRBs at z > 4 by a factor
of ~2-3 and find that, in contrast to samples at mostly lower redshift, all of
the GRB afterglows have a visual extinction of < 0.5 mag. Analysis of the
GROND detection thresholds and results from a Monte-Carlo simulation show that,
although we partly suffer from an observational bias against highly
extinguished sight-lines, GRB host galaxies at 4 < z < 6 seem to contain on
average less dust than at z ~ 2. Additionally, we find that all of the GRBs can
be modeled with locally measured extinction curves and that the SN-like dust
extinction curve provides a better fit for only two of the afterglow SEDs. For
the first time we also report a photometric redshift of for GRB
100905A, making it one of the most distant GRBs known to date.Comment: 26 pages, 37 figure
The environment of the SN-less GRB 111005A at z = 0.0133
The collapsar model has proved highly successful in explaining the properties
of long gamma-ray bursts (GRBs), with the most direct confirmation being the
detection of a supernova (SN) coincident with the majority of nearby long GRBs.
Within this model, a long GRB is produced by the core-collapse of a metal-poor,
rapidly rotating, massive star. The detection of some long GRBs in metal-rich
environments, and more fundamentally the three examples of long GRBs (GRB
060505, GRB 060614 and GRB 111005A) with no coincident SN detection down to
very deep limits is in strong contention with theoretical expectations. In this
paper we present MUSE observations of the host galaxy of GRB 111005A, which is
the most recent and compelling example yet of a SN-less, long GRB. At
z=0.01326, GRB 111005A is the third closest GRB ever detected, and second
closest long duration GRB, enabling the nearby environment to be studied at a
resolution of 270 pc. From the analysis of the MUSE data cube, we find GRB
111005A to have occurred within a metal-rich environment with little signs of
ongoing star formation. Spectral analysis at the position of the GRB indicates
the presence of an old stellar population (tau > 10 Myr), which limits the mass
of the GRB progenitor to M_ZAMS<15 Msolar, in direct conflict with the
collapsar model. Our deep limits on the presence of any SN emission combined
with the environmental conditions at the position of GRB 111005A necessitate
the exploration of a novel long GRB formation mechanism that is unrelated to
massive stars.Comment: Now accepted by A&A. Manuscript replaced to match accepted version.
Some additional discussion added, and velocity map of the host galaxy now
include
GRB afterglow light curves from realistic density profiles
The afterglow emission that follows gamma-ray bursts (GRBs) contains valuable
information about the circumburst medium and, therefore, about the GRB
progenitor. Theoretical studies of GRB blast waves, however, are often limited
to simple density profiles for the external medium (mostly constant density and
power-law R^{-k} ones). We argue that a large fraction of long-duration GRBs
should take place in massive stellar clusters where the circumburst medium is
much more complicated. As a case study, we simulate the propagation of a GRB
blast wave in a medium shaped by the collision of the winds of O and Wolf-Rayet
stars, the typical distance of which is d /sim 0.1 - 1 pc. Assuming a spherical
blast wave, the afterglow light curve shows a flattening followed by a shallow
break on a timescale from hours up to a week after the burst, which is a result
of the propagation of the blast wave through the shocked wind region. If the
blast wave is collimated, the jet break may, in some cases, become very
pronounced with the post break decline of the light curve as steep as t-5.
Inverse Compton scattering of ultra-violet photons from the nearby star off
energetic electrons in the blast wave leads to a bright \simGeV afterglow flare
that may be detectable by Fermi.Comment: 7 pages, 7 figures, submitted to MNRA
A strong optical flare before the rising afterglow of GRB 080129
We report on GROND observations of a 40 sec duration (rest-frame) optical
flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a
power law with indices +12 and -8, respectively, inconsistent with a reverse
shock and a factor 10 faster than variability caused by ISM interaction.
While optical flares have been seen in the past (e.g. GRB 990123, 041219B,
060111B and 080319B), for the first time, our observations not only resolve the
optical flare into sub-components, but also provide a spectral energy
distribution from the optical to the near-infrared once every minute. The delay
of the flare relative to the GRB, its spectral energy distribution as well as
the ratio of pulse widths suggest it to arise from residual collisions in GRB
outflows \cite{liw08}.If this interpretation is correct and can be supported by
more detailed modelling or observation in further GRBs, the delay measurement
provides an independent, determination of the Lorentz factor of the outflow.Comment: accepted for publ. in ApJ, 5 Fig
PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe
We present the results of a multifrequency campaign on the high-redshift (z =
3.1) blazar PKS 0537-286. The source was observed at different epochs from 2006
to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based
near-IR/optical telescopes. The SEDs are compatible with a model based on
synchrotron radiation and external inverse Compton scattering. The campaign
gives an insight into the physical environment of the blazar.Comment: 8 pages, 5 figures, accepted for publication in A&
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